Periods: Antecedent and Consequent

Neither phrase in a period sounds finished by itself. The antecedent phrase sets up melodic and harmonic material and then stops on a half cadence, a pause on the dominant that feels like a comma, not a period. That open ending is the whole point: it tells your ear the thought isn't done. The consequent phrase then answers, usually reusing the same opening melodic idea, but pushes further and lands on an authentic cadence, a full close on the tonic. Hearing the weaker half cadence resolve into the stronger authentic cadence is what fuses two separate phrases into one unified statement.

The cadences have to escalate in strength, not just differ. An antecedent ending on a half cadence followed by a consequent ending on an authentic cadence creates that question-answer logic. If both phrases ended on half cadences, or both on authentic cadences with no clear hierarchy, you'd have a phrase group instead of a period: two phrases sitting side by side without the directed motion toward a stronger close. The matching phrase lengths also matter. If the antecedent is four measures, the consequent is almost always four measures too, giving a symmetrical eight-measure period rather than an uneven pair.

A period is itself a building block. Once two phrases lock together as antecedent and consequent, that whole eight-measure unit can function as one phrase group inside a larger binary or ternary form, pairing with another period or a contrasting section to build the full piece.